Introduction
Asphalt pavement, comprising asphalt coated particles bound by the asphalt, is known to be highly porous. The porosity exists in the form of pores connected through capillary channels formed in part during the compaction process. The pores and channels are affected by variations in aggregate size, and are formed in part because of an entrapment of solvent during the curing process. Fatigue caused by expansion and contraction due to heat variation also creates gaps between particles within a pavement matrix. These gaps may develop into cracks if not treated. The oxidation process of asphalt coated particles and the exposure to UV light are also known to cause further damage to bonds between asphalt and aggregate, which damage increases porosity as aggregates at the surface become loose.
The presence of pores and capillaries allows water penetration, a phenomenon that causes additional damage to asphalt pavement. Water reduces the bonding strength between the asphalt and the gravel or any other material under the pavement. Water penetration allows the penetration of chloride ions from deicing salts, a chemical that attacks the asphalt matrix and shortens its life. In addition, freeze and thaw spalling and chipping becomes a problem in asphalt pavement in cold climates because of the fatigue and internal stress build-up due to the expansion of water upon freezing.
More Particularly Asphalt and Water
Water penetration through asphalt pavement may cause severe damage to the bonding strength between asphalt and aggregate. Water penetrates because of its unimpeded ability to move freely through capillaries and connected pores and voids. Typically, asphalt pavement is 13% to 20% voids. The typical aggregate to binder ratio is 10/1.
Because of its ability to move freely through capillaries and connected pores and voids, water causes severe damage to asphalt pavement by several mechanisms. Water or moisture results in a breaking of the bonds between asphalt particles and aggregates. This in turn results in a weakening of the pavement and making it susceptible to problems that lead to a loss in strength and durability. Detachment, wherein a thin film of water results in the separation of an asphalt film from an aggregate surface without breaking the bond, has a high potential because of the ability of water to wet the aggregate surface more than the asphalt binder, due its lower surface tension. This phenomenon generally starts at the surface of the pavement and gradually moves downward as it develops to displacement, a condition where the asphalt film ruptures and the bonds between the asphalt and the aggregate break, which may appear in the form of loose aggregates. See references.
Under wet conditions, repeated traffic and load applications result in the entrapment of water inside tiny pores. The entrapment leads to distress and continued buildup in pore pressure resulting in disrupting the asphalt film from the aggregate surface, which causes the formation of cracks.
In cold climates, where repeated cycles of freezing and thawing occur, asphalt pavement with sufficient moisture is particularly susceptible to additional damage. When the temperature drops below the freezing point ice starts to form within the pores and capillaries of the pavement. Since water volume increases by 9% upon on freezing, if water is confined in the pores between freezing bodies and placed under compression, the pores may dilate causing an increase in the internal stress against the surrounding pavement particles. Repeated freeze and thaw cycles can result in the rupture and deterioration of the asphalt pavement due to fatigue stresses. Such deterioration may appear in the form of cracks and surface spalling. With time, fatigue stress can cause big chunks of the pavement to pop out.
The penetration of water can be greatly influenced by the use of de-icing salts such as sodium chloride granules in cold climates. The concentration of such material within the pavements pores and voids increases with time. The result is an increase in the osmotic pressure, allowing more water to be absorbed under wet conditions at moderate temperatures.
Oxidation of Asphalt
The rate of oxidation of asphalt pavement is highly dependent on the voids in the total mixture. If the voids in the total mixture can be brought below 7–8% in-place, however, then the effect of oxidation will be greatly minimized. During the oxidation reaction, asphalt loses a significant amount of its saturate and aromatic components, which causes the asphalt mixture to stiffen at low temperatures, resulting in further crack formation.
Current Art Techniques
Maintenance of most asphalt pavements involves repairing localized problem areas, such as potholes or badly cracked pavement sections, and in sealing cracks. This type maintenance is needed to prolong the pavement life and to prevent rapid damage to the pavement due to water penetration and other causes. Some problems with asphalt pavement can be prevented or delayed by using good maintenance practices. Currently, there are three different maintenance methods commonly used: rejuvenators, slurry seals, and surface treatments. The choice between the methods mainly depends on the specific project to be maintained.
Asphalt sealers currently available in the market are typically intended for use on low traffic asphalt pavement as a protective seal coat of a film-forming nature, which sealcoat acts as a “barrier coat” to protect the asphalt surface. There are two primary types: those made from refined coal tar and those made from asphalt. Refined coal tar—a by-product of the coking process—is complex mixture of thousands of chemicals and has different molecular structure in general from asphalt. The coal tar molecules have a predominantly closed ring (aromatic) structure with a minor degree of un-saturation. Because of their stable molecular structure, the destructive elements of weather and chemicals do not particularly affect the properties of coal tar. Sealcoatings based on a refined coal tar were introduced in the 1950s and until recently have been used extensively to protect off-street pavements. These sealcoatings often are referred to as C.T.P.E (Coal Tar Pitch Emulsions,) denoting that these coatings are water based, obtained by dispersing refined coal tar in a matrix of clay and water. In recent years, asphalt emulsion-based coatings have been introduced with varying degrees of success. In fact, many sealer manufacturers that previously produced only refined coal tar sealers now also produce asphalt-based sealers or even asphalt/refined coal tar blends. The asphalt emulsion based coatings deliver most of the same properties as refined coal tar-based coatings—except for a resistance to color fading due to ultraviolet degradation and for a resistance to salts and petrochemicals like oils, fats, grease and solvents. These deficiencies are inherent in the asphalt binder itself. Being a petroleum derivative, asphalt has a natural affinity for petrochemicals, so it is easily dissolved by them. Asphalt emulsion-based coatings are made using either a soap emulsion (SS-1-H, for example) or a clay stabilizing emulsion. In recent years, asphalt sealer manufacturers have been quite successful in refining the performance of asphalt emulsion based sealers by using specialty chemicals and pigments. However, the asphalt emulsion-based coatings resistance to petrochemicals and solvents—while improved—has yet to be overcome.
Silicone-based chemicals have been tested and used as additives to asphalt products to enhance the bonding properties between aggregates especially in cold applied patching and repair materials. Ward, Jr. (U.S. Pat. No. 4,373,960, U.S. Pat. No. 4,453,980, and U.S. Pat. No. 4,479,827) utilizes an organopolysiloxane material with non-emulsified asphalt to produce an asphalt-based binder that is to be mixed with pre-heated aggregates prior to application as a patching material for deteriorated pavements. In his inventions, the organopolysiloxane was at most 0.05% by weight, sufficient to enhance the products free flowing properties.
A special blend of topped-coke-oven tar and aromatic solvent was introduced by McGoven (U.S. Pat. No. 4,661,378) as a penetrating sealer and rejuvenator for deteriorated asphalt pavements as well as for concrete surfaces. McGoven claimed that such material might penetrate up to 0.4 inch into asphalt pavement when applied on low-traffic pavement at a rate of 0.13 gallon/square yard. However, for heavy traffic asphalt pavement, such as roads, it had to be mixed with sand, pozzolana, or other fine mineral aggregates, which makes a slurry coat having more body than desired as in the case of conventional slurry-seal materials. A similar form of surface treatment consisting of an asphalt emulsion, diatomite, and sand that can be applied under ambient temperature using conventional paving machinery was invented by Kietzman (U.S. Pat. No. 4,548,650), where the filler diatomite to asphalt ration is in the range of 0.008 to 0.3 by weight. In addition to its overlay uses, Kietzman claimed that this material (with a little modification to improve its abrasion resistance, adhesion/cohesion, and tensile strength) could be used as a protection membrane for bridge decks and roads.
In summary, conventional asphalt sealers currently available in the market have several defects. They are typically surface treatments. In addition to a lack of providing internal protection due to the high viscosity of the surface treatments, which does not allow them to penetrate, they may be considered a non-permanent treatment since they tend to wear-off the surface because of traffic. Because of their film-forming nature combined with their tendency to remain on the surface, these surface treatments cannot be used on roads and highways where slipperiness and skid resistance are of great concern unless they are broadcast with fine aggregates while wet or pre-mixed with fine aggregate in slurry form. This makes the treatment process itself less economical, due to the low coverage rate and frequent shut-down times.
Sealing heavy-traffic asphalt pavements with a penetrating sealer, including an oxidized asphalt cutback that has been modified with a silicone-based compound that permanently provides internal as well as surface protection, to make a heavy-traffic asphalt pavement more durable, has never been taught, disclosed or practiced to applicant's knowledge, prior to the instant invention. There is a need for a new technology that more thoroughly addresses treatment problems for asphalt pavement in a cost-effective mater.